Our laboratory has been obtaining pure and defining a number of bacterial enzymes of interest because of complex regulatory phenomena. We have been generally investigating the glutamine synthetase of Escherichia coli which is subject to a cumulative feedback inhibition involving a number of different sites. Recent activity has centered around the effect of trace metal additions to the medium on the quantity and quality of enzyme produced. These studies are providing possible explanations for variations in the sensitivity of different enzyme preparations and will be extended to the investigation of a collection of temperature sensitive mutants with defective glutamine synthetase activities. We have also been investigating the possibility of flexible catalytic sites in comparing the substrate specificity patterns of the xanthine dehydrogenases of several bacteria and of 2-oxypurine dehydrogenase of Micrococcus aerogenes. The Glutamic dehydrogenase of M. aerogenes has also been studied and has been shown to constitute up to 10 per cent of the total bacterial protein. This enzyme shows unusual specificity with regards to the reversibility of the reaction using different pyridine nucleotides. With the aid of a mutant of Escherichia defective with regard to the cytoplasmic asparaginase we have recently succeeded in defining several systems which are responsible for the transport of l-asparagine by this organism. I have also been recently investigating the conditions which bring about the release of l-asparaginase activity into the external medium by a bacterium isolated in this laboratory and capable of producing high levels of asparaginase activity. We have also been investigating the process of enzyme secretion by Streptomyces through a careful comparison of the chemical features of external and internal proteins and an investigation of chemical agents specific in their action with regard to the production of these two classes of protein.